This invention relates to variable gain amplifiers, and in particular to a differential variable gain transconductance amplifier suitable for use in voice frequency circuits and especially adapted for operation with very low power supply voltage.
Voice frequency amplifiers are widely used in telephone circuitry and variable gain amplifiers are desired in some applications. A variety of amplifier circuits have been utilized for varying gain, and two such circuits are shown in U.S Pat. Nos. 3,786,200 and 4,536,888. Other circuits of this type are shown in the following integrated circuit catalog sheets: Fairchild A706, A733, and A757, Signetics NE/SE592, Motorola Linear Integrated Circuit Data Book, Dec. 1971 MC 1590G; the National Semiconductor publication Linear Applications, Feb. 1973, pages AN15-2, AN31-9, AN31-15, AN32-5 and LB1-2; and RCA Application Note ICAN-6668 pages 411-413.
One important characteristic of all amplifiers is the magnitude of the power supply voltage required for operating the circuits. It is an object of the present invention to provide a new and improved variable gain amplifier suitable for use with voice frequency circuits which can operate with a voltage source of less than one volt. The specific circuit described herein is designed for operation with a voltage supply 1.3 volts, readily available from a 1.5 volt battery. However the circuitry of the invention can be operated with a supply voltage as low as 0.9 v at 25.degree. C. As junction temperature decreases, the lower operation supply voltage limit rises .apprxeq.2.2 mv/.degree.C. due to increasing base-emitter voltages.
Other objects, advantages, features and results will more fully appear in the course of the following description.